The present invention relates to industrial vehicles having body frames that are tiltably supported by axles, and more particularly, to hydraulic circuits for controlling the tilting of axles.
In the prior art, forklifts having body frames that are tiltably supported by axles have been proposed to improve the driving performance and comfort of the vehicle. If such a forklift is steered to change directions, lateral acceleration (centrifugal force) acts on the vehicle and tilts the forklift. This destabilizes the steering performance of the forklift and prevents the speed of the forklift from being increased.
Japanese Unexamined Patent Publication No. 58-211903 describes a forklift that has a turn detector for detecting centrifugal force produced when the vehicle changes directions. If the detected centrifugal force exceeds a predetermined value, a locking mechanism locks the axle to the body frame and stabilizes steerage of the forklift.
A damper is arranged between the body frame and the axle to lock the axle to the body frame. Hydraulic oil is communicated to the damper through an oil passage. The damper locks the axle when the oil passage is closed and the flow of oil to the damper is stopped. To unlock the axle, the oil passage is opened. This permits the axle to be tilted with respect to the body frame.
Expansion of the hydraulic oil may result in oil leakage that decreases the hydraulic pressure of the oil passage. However, the hydraulic oil is not replenished in the above prior art forklift despite the oil leakage. This may degrade the damper locking effect that prohibits relative movement between the body frame and the axle.
To maintain the desired damper locking effect, the hydraulic fluid must be replenished periodically to guarantee adequate pressure in the hydraulic circuit. Thus, maintenance of the hydraulic circuit is burdensome.